Page:Collected Physical Papers.djvu/179

Rh (fig. 31) it is seen that the reaction of a negative substance is in every respect opposite to that of a positive substance. It will be noticed that in confirmation of theory, heating or tapping produces restoration of sensitiveness in the first class by an increase, and in the second by a decrease of resistance. I have been able to verify this deduction by observations with nearly a dozen different substances, and have not, so far, come across any to contradict it. It thus appears that tapping restores the sensitiveness not by the separation of the electrically-welded particles (in which case tapping ought to have produced an increase of resistance in both the classes of fatigued substance), but by removing the strain in B and thus converting it into A.

The effect of electric radiation is thus to produce rearrangement of atoms and molecules in a substance;

so does light produce new atomic and molecular aggregation in a photographic plate, a subject to be dealt with in detail in a future paper. Some of my audience at the Royal Institution (January, 1897) may remember